The timbuk2 hemlock is a lot like my timbuk2 grubstake. Same strap design and same back. You will sweat. It's like a big black blanket covering your back.

I was considering the hemlock when looking for a bag a few weeks back and didn't pick it for a few reasons. There are a few reviews on the hemlock and they all say the outer pocket is worthless. Only a small velcro secures it and when the bag is full it doesn't close, and if it does, barely. Also no waterproofing, and the rolltop will block your view on the bike.